A gas turbine generally includes a compressor section, a combustion section having a combustor and a turbine section. The compressor section progressively increases the pressure of the working fluid to supply a compressed working fluid to the combustion section. The compressed working fluid is routed through and/or around a fuel nozzle that extends axially within the combustor. A fuel is injected into the flow of the compressed working fluid to form a combustible mixture. The combustible mixture is burned within a combustion zone to generate combustion gases having a high temperature, pressure and velocity. The combustion gases flow through one or more liners or ducts that define a hot gas path into the turbine section. The combustion gases expand as they flow through the turbine section to produce work. For example, expansion of the combustion gases in the turbine section may rotate a shaft connected to a generator to produce electricity.
The temperature of the combustion gases directly influences the thermodynamic efficiency, design margins, and resulting emissions of the combustor. For example, higher combustion gas temperatures generally improve the thermodynamic efficiency of the combustor. However, higher combustion gas temperatures may increase the disassociation rate of diatomic nitrogen, thereby increasing the production of undesirable emissions such as oxides of nitrogen (NOx) for a particular residence time in the combustor. Conversely, a lower combustion gas temperature associated with reduced fuel flow and/or part load operation (turndown) generally reduces the chemical reaction rates of the combustion gases, thereby increasing the production of carbon monoxide (CO) and unburned hydrocarbons (UHCs) for the same residence time in the combustor.
In order to balance overall emissions performance while optimizing thermal efficiency of the combustor, certain combustor designs include multiple fuel injectors that are arranged around the liner and positioned generally downstream from the primary combustion zone. The fuel injectors generally extend radially through the liner to provide for fluid communication into the combustion gas flow field. This type of system is commonly known in the art and/or the gas turbine industry as Late Lean Injection (LLI) and/or as axial fuel staging.
In operation, a portion of the compressed working fluid is routed through and/or around each of the fuel injectors and into the combustion gas flow field. A liquid or gaseous fuel from the fuel injectors is injected into the flow of the compressed working fluid to provide a lean or air-rich combustible mixture which spontaneously combusts as it mixes with the hot combustion gases, thereby increasing the firing temperature of the combustor without producing a corresponding increase in the residence time of the combustion gases inside the combustion zone. As a result, the overall thermodynamic efficiency of the combustor may be increased without sacrificing overall emissions performance.
One challenge with injecting a fuel into the combustion gas flow field using existing LLI or axial fuel staging systems is that the momentum of the combustion gases generally inhibits adequate radial penetration of the liquid fuel into the combustion gas flow field. As a result, local evaporation of the liquid fuel may occur along an inner wall of the liner at or near the fuel injection point, thereby potentially resulting in a high temperature zone and/or high thermal stresses. In addition, achieving and sustaining combustion in a gas turbine combustor is difficult due to various factors such as but not limited to fuel content, fuel temperature, ambient air conditions, engine load and/or operating condition of the gas turbine. These various factors may potentially create flow instabilities which may affect the NOx emissions levels generated by the combustor.
Current solutions to address these issues include extending at least a portion of the fuel injector radially inward through the liner and into the combustion gas flow field. However, this approach exposes the fuel injectors to the hot combustion gases which may potentially impact the mechanical life of the component and may lead to fuel coke buildup. Therefore, an improved system for injecting a combustible mixture into the combustion gas flow field including a trapped vortex fuel injector disposed downstream from a primary combustion zone and method for fabricating the trapped vortex fuel injector would be useful.